Not Applicable
The present invention relates to the field of film passive microwave devices such as passive terminations.
It has been known to make microwave terminations by forming a single film resistive element on a substrate such as ceramic. In the case of a 50-ohm termination, for example, a resistive element can take the form of a square of thick-film material having resistivity of 50 ohms per square. A square shape is often desirable for manufacturing reasons. It is therefore common to use a film material whose resistivity is exactly equal to the desired termination resistance, so that such material can be deposited in the desired square shape and provide the desired termination resistance.
When a non-sinusoidal microwave signal is applied to a film termination in an operating system, some degree of intermodulation distortion may be added to the signal by the device. Such intermodulation distortion arises from the non-linearity of the characteristics of the device with respect to the voltage of the applied signal. It is desired to minimize any such intermodulation distortion in an operating system.
In accordance with the present invention, a film microwave termination is disclosed. The termination exhibits lower intermodulation distortion than typical prior art terminations formed from a single film resistive element.
The disclosed termination device includes a substrate, such as a ceramic substrate, and multiple film resistors formed on the substrate. The film resistors each have a predetermined resistivity generally less than a desired characteristic impedance of the device. The resistors are connected together in series so as to (i) provide an overall impedance equal to the desired characteristic impedance, and (ii) generate less intermodulation distortion in microwave signals coupled to the termination device than would be generated by an alternative termination device employing a single film resistor whose resistivity is equal to the desired characteristic impedance. In an illustrated embodiment for a 50-ohm termination, four square elements are formed of thick-film material having resistivity of 12.5 ohms per square, and these four elements are connected in series on the substrate to realize the desired 50 ohm termination impedance. Other common system impedances such as 12.5 or 100 ohms can be realized in a similar fashion using corresponding lower or higher resistivity material. Alternative embodiments may employ thin-film rather than thick-film material.
It is believed that the use of multiple elements of lower resistivity exhibits better linearity with respect to voltage, and therefore reduced intermodulation distortion, than exhibited by a traditional single-element termination. It is believed that this improved linearity arises from the lower sheet resistivity of the material used to make the resistors.
Other aspects, features, and advantages of the present invention will be apparent from the detailed description that follows.